I have totally given up trying to dual boot between XPpro/SP2 32 bit and Vista Ultimate 64 bit
I am now in the process of building a new desktop which will have plug-in hard drive bay. One HDD I plug in will have the full Vista Ultimate 64 bit and the other HDD will have the normal XPpro/Sp2 setup (which I will be transferring from my current computer.
Incidentally, some time ago I tried dual booting with two physically separate drives between 32 bit and 64 bit XPpro and had very similar difficulties and no sensible help from M$.